1. Field of the Invention
This invention relates to the field of semiconductor devices and more particularly to a method for forming a thin, high integrity, silicon dioxide layer. The thin SiO2 layers formed by the present invention are ideal for use as a gate oxide.
2. Prior Art
In the semiconductor industry, silicon dioxide (SiO2) films are used in a variety of applications. Often they are used as a dielectric or insulative layer to separate electrically various regions or structures. Examples of use as an insulative layer include as a gate oxide, as an interlevel dielectric between metal 1 and metal 2, and as field isolation. SiO2 is also used for scratch protection and passivation purposes.
When used as a gate oxide on an MOS device, the SiO2 layer is disposed above the source, drain, and channel regions of the silicon substrate, with the gate of the device formed on the SiO2 layer. The gate oxide thus electrically insulates the gate from the source and drain.
When used as a field isolation, a field oxide is formed to insulate electrically one device, for example a transistor, from another. Traditionally local oxidation of silicon (LOCOS) is used to form the field isolation. Active regions of the silicon substrate are covered with a mask such as silicon nitride, while the field regions remain exposed to an oxidizing ambient to form the field oxide. Recently, advanced isolation techniques are being used on MOS devices in place of LOCOS technology. Various recessed isolation technologies are used to improve device performance. For example, the recessed sealed sidewall oxidation technique (RESSFOX). In this technique, what will become the field regions are first etched while the device areas of the substrate remain covered. The side walls of the recessed regions are also covered with the same masking material as the device regions, commonly silicon nitride with an underlying thermal pad oxide. The advanced isolation techniques offer less lateral encroachment of the field oxide into the active regions (commonly known as the bird""s beak) as well as a more planar surface than conventional LOCOS technologies. One drawback of these advanced techniques is that sharper edges are formed on the substrate surface. These sharp edges are more difficult to oxidize in the later oxidation step for forming the gate oxide. An additional problem with many of the recessed technologies is the requirement of a silicon etch in the field oxide region prior to field oxide growth. The silicon etch creates contamination which remains on the wafer during subsequent steps. Thus, contamination from the silicon etch may lead to defects in the subsequently grown gate oxide. For an in-depth discussion of conventional and advanced isolation techniques, see Silicon Processing For The VLSI Era, by Stanley Wolf, Volume 2, Chapter 2, pp 12-83 (Lattice Press 1990).
SiO2 can be deposited by such techniques as sputter deposition or chemical vapor deposition (CVD) directly on the substrate. SiO2 can also be grown by oxidizing exposed silicon. SiO2 can be grown in a xe2x80x9cdryxe2x80x9d process utilizing oxygen (O2), or in a xe2x80x9cwetxe2x80x9d process using steam as the oxidizing agent. Gate oxides are typically grown as opposed to deposited.
Because SiO2 layers electrically isolate active device regions, the integrity of the oxide film has a large impact on device performance. Also, the scaling of device dimensions to enhance circuit density and speed performance requires the scaling of oxide thickness. For example, a 5.0 v, 0.8xcexc technology requires an oxide thickness of about 150 xc3x85 for high performance, while a 3.3 V, 0.5xcexc technology requires an oxide thickness of approximately 70-80 xc3x85 for high performance. Therefore, the ability to form a high quality, low defect SiO2 film has become increasingly important. Such thin gate oxides are particularly important for devices with RESSFOX isolation. In RESSFOX devices, the minimization of bird""s beak encroachment into the active regions has allowed for scaling of device dimensions. Also, the planar surface of these devices allows for higher resolution lithography. Because of the scaling of device dimensions achievable with RESSFOX, a thin gate oxide is necessary. One measurement of the quality of an SiO2 film is the current density it can withstand without breakdown, known as Jt or change-to-breakdown. Generally, an SiO2 film used as the gate oxide must be able to withstand a ramp Jt of 1 Coulomb per square centimeter (1 C/cm2) or greater when measured on large area MOS capacitors (e.g. area=0.0695 cm2).
In any SiO2 growth or deposition, process contamination can lead to unacceptable SiO2 layers. The contamination can be in the form of particulate matter or ionic contamination such as sodium ions (Na+). While a wet process is generally more successful in oxidizing the sharp edges of features such as those which occur on devices with advanced isolation technologies, wet processes generally exhibit a higher defect density than dry oxidation processes. Often, to reduce defects in the film, a small amount of chlorine is included along with the oxidizing agent in order to clean up the surface and reduce the defect density of the grown film. The chlorine is usually added to a dry oxidation step since many chlorine containing compounds do not reach in steam to form Cl, the necessary species for wafer cleaning. Usually, the chlorine concentration is limited to about 1% to 3% of the total gas volume in the oxidizing mixture, because excess chlorine may become entrapped in the oxide, making it more susceptible to high-field hot electron damage and, therefore, less reliable. A process for growing a gate oxide of 175 xc3x85 using dry, dilute oxygen oxidation, a steam with chlorine (Cl2) oxidation, and a final dry dilute oxygen oxidation is described in F. Bryant and F.T. Liou, Proc. Electrochemical Soc. Volume 89-7, pp. 220-228 (1998). The process and properties of a 175 xc3x85 steam oxide (without chlorine) is described in C. Y. Wei, Y. Nissan-Cohen, and H.H. Woodbury, IEEE Trans Electron Devices, Volume 38, No. 11, November 1991, pp. 2433-2441. Other processes for growing oxides using chlorine or chlorine containing compounds such as anhydrous hydrogen chloride (HCl), trichloroethylene (TCE), and trichloroethane (TCA) are described in Silicon Processing For The VLSI Era, Volume 1, Chapter 7, pp 215-216.
What is needed is a process for growing a high integrity oxide film. The oxide film should exhibit reduced defects and effective oxidation of sharp edges, allowing for high reliability of devices fabricated utilizing advanced isolation techniques. It is further desirable that the oxide formed be sufficiently robust to allow for thin oxide layers for use as a gate oxide in sub-micron VLSI applications.
A process for fabricating a high integrity silicon dioxide layer is described. The oxide formed can be used as a sub-100 xc3x85 gate oxide. Since the oxide shows low defects and effectively oxidizes sharp silicon corners or features, it is particularly well suited for use on devices with advanced isolation technologies utilizing recessed field oxides.
First, during wafer push, pure nitrogen is flowed over the substrate to limit native oxide growth. During temperature ramp and stabilization, 1% oxygen in nitrogen flows through the furnace to form a tightly controlled native oxide layer of approximately 5-10 xc3x85.
Next, two low temperature oxidation steps are performed to grow the oxide layer. First, a dry oxidation in 13% trichloroethane (TCA) is performed. In this step, the high concentration of TCA cleans up the surface allowing for a low defect oxide layer. During this step, the silicon surface is protected by the native oxide grown during temperature ramp and stabilization.
Then, a wet oxidation in pyrogenic steam is performed. This oxidation is efficient in oxidizing the sharp features associated with recessed field oxides. It also depletes the chlorine (which is incorporated in the oxide during the 13% TCA dry oxidation) so that the final oxide is essentially chlorine-free.
After a final stabilization and temperature ramp down in a pure N2 flow, the wafers are pulled from the furnace. In the currently preferred embodiment, the total thickness of the oxide layer is 60-80 xc3x85.
In another embodiment, for oxides thicker than 100 xc3x85, 40 xc3x85-90 xc3x85 of the novel oxide as described above is grown. A deposited oxide is then added to make up the final thickness of more than 100 xc3x85. The combination oxide stack, known as composite oxide, gives much lower defect density than a standard chlorinated thermal oxide for thicknesses over 100 xc3x85.